The present disclosure relates to manual control devices, such as joysticks. More specifically, the present disclosure relates to a manual control device including a magnetoresistive sensor element.
Manual control devices, commonly referred to as joysticks, are used in various apparatus such as video games, heavy construction equipment and aircraft to control parameters such as position, velocity and acceleration. Typically, the joystick has an extended length shaft with a handle at one end and a shaped component at the opposing end that interacts with one or more sensors. Movement of the handle is translated by the sensors into electrical signals that are communicated to the apparatus actuating a desired response.
In one type of joystick, movement of the handle displaces one or more electric potentiometers changing the voltage output. While the potentiometers provide accurately defined signals to the apparatus, the joystick shaft is mechanically coupled to the potentiometers through linkages and gearing. The connection deteriorates over time due to potentiometer and linkage wear. This type of joystick has a large number of moving parts and is subject to premature failure in robust operating environments.
A different type of joystick is disclosed in U.S. Pat. No. 3,331,972 by Möller, that is incorporated by reference in its entirety herein. The Möller patent discloses a joystick having an extended length shaft with a handle at one end and a joint ball at the other end. The joint ball has an embedded magnet that is surrounded by bands of a ferromagnetic material. Movement of the joystick handle completes a magnetic circuit. A number of Hall effect sensors, semiconductor devices that generate a voltage when engaged by a magnetic flux, circumscribe the magnet. Movement of the joystick handle changes the magnetic flux lines, generating a voltage in the Hall effect sensors.
This Hall effect joystick is more robust than a potentiometer-type joystick. The joint ball does not mechanically engage the sensors reducing the risk of mechanical degradation. However, due to inclusion of ferromagnetic components, hysteresis degradation is a problem. As the ferromagnetic components become slightly magnetized, the resulting magnetic field affects the sensors, changing the characteristics of the joystick.
U.S. Pat. No. 5,850,142 to Rountos et al. is directed to an angle measurement device (e.g., a joystick) capable of providing both linear and non-linear control transfer functions directly using a shaped, generally convex, magnet and linear magnetic sensors, such as Hall sensors. This joystick uses non-contacting Hall effect sensors to provide the X and Y-axis output signals. Advantageously, the shape of the magnet is effective to provide a desired transfer function, and the magnet is constructed to minimize magnetic flux distortion, cross talk and hysteresis.
The use of magnetic sensors and magnets in joystick controls requires a careful study of their susceptibility to external magnetic fields and nearby magnetic material such as iron and steel. For example, Hall effect sensors operate by providing an output voltage that is proportional to the intensity and sign of the applied magnetic field. External magnetic fields and nearby magnetic material can affect the Hall effect sensors, changing the characteristics of the joystick.
For certain applications, the sensors can be positioned away from any external magnetic fields and magnetic materials to avoid their interference. For example, in the joystick described in U.S. Pat. No. 5,850,142, the X and Y-axis Hall effect sensors are located in the base of the assembly, which may be mounted below a panel in the final installation. The Hall sensors and the associated magnet assembly are thus located some distance away from the operator and any steel tools or other steel or magnetic objects the operator might bring close to the joystick. Generally, iron or steel objects placed 4 inches or more away from the Hall sensors in the base of the joystick will not interfere with the normal operation of the joystick.
However, the use of Hall effect sensors for a twist axis mounted above the base assembly would not provide the required magnetic immunity. This sensor assembly would be exposed to nearby objects on the control panel or objects that may be temporarily placed on the control panel by the operator. As stated above, the Hall sensors are sensitive to magnetic field intensity so a nearby magnet or a steel tool will modulate and distort this field intensity and cause spurious output signals from this twist axis sensor.
Thus, there remains a need for a joystick that provides increased magnetic immunity, particularly for twist axis sensors which, because of their relatively unshielded position, are more susceptible to external magnetic fields.